Filter arrangement for symmetrical and assymmetrical line systems

ABSTRACT

A filter arrangement includes one first resonator circuit connected between a first node and a second node. A second resonator circuit is connected between a third node and a fourth node. The first resonator circuit and the second resonator circuit are intercoupled. Further, an inductive device is provided, connected between the first node and the third node.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a filter arragement and inparticular to a resonator band-pass filter arrangement for balanced andunbalanced line systems.

[0003] 2. Description of the Prior Art

[0004] Band-pass filters are needed in almost any microwave application.In particular narrow-band transmit/receive circuits (so calledtransceiver-circuits), as they are used in mobile radio communicationsystems, need band-pass filters in order to suppress all interferencesignals from outside the used frequency band. Thus, large interferencesignals cannot limit the receiver and small interference signals do notdeteriorate the basic noise. Further, band-pass filters are used inso-called mutiband systems in order to select the individual bands overfrequency-separating filters, so-called diplexers.

[0005] Good band-pass filters distinguish themselves by showing verylittle electrical losses in the passband and having as much isolation aspossible in the stopband and/or blocking band.

[0006] Frequently, resonators are used in band-pass filters. A serialresonator is concerned, if this two-terminal element shows a very goodpassband performance with the resonance frequency, if connected inseries. For other frequencies this element shows a blocking performance.A parallel resonator is concerned, if this two-terminal element shows avery good passband performance with the resonance frequency if connectedbetween the signal path and a reference potential, like e.g. ground. Forother frequencies this element shows a blocking performance.

[0007] A balanced transformer (Balun) is used, where a transition froman unbalanced to a balanced line system, e.g. a microwave line system,or the contrary is necessary. In modern handset devices small signaltransmitter/receiver circuits (small signal transceiver) integrated insemiconductors exclusively have balanced inputs and outputs. However,power amplifiers and antennas are implemented in an unbalancedtechnique. Therefore, two networks are required for every frequency bandin such handset devices in order to realize low-loss transitions betweenthe balanced power system and the unbalanced power system. In the mobileradio area unbalanced circuits, like e.g. diplexers, are theconventional and only used circuits so far. Further, a large number ofline systems used in practice are unbalanced systems, like e.g. coaxial,microstrip and strip lines.

[0008] For realizing band-pass filters a large numbers of realizationpossibilities exist.

[0009] For many decades filters having concentrated inductive membersand capacities or having line structures have been known. Infrequency-separating filters (diplexers) usually pure line structuresare used.

[0010] For narrow-band filter applications surface acoustic wavefilters, the so-called SAW-filters, have been used for years, in whichnon-coupled resonators are used. Such non-coupled resonators are forexample arranged in the so-called ladder-type structure.

[0011] For narrow-band band-pass filters realizations with coupledresonators have only become established for circuits in the mobile radioarea in the last few years. These filter realizations especiallydistinguish themselves by their very steep filter edges. This technologyfinds its use with large resonators, e.g. of cylindrical ceramics withlow losses and a large dielectric constant and with filters for greatpowers, like they are for example used in base stations.

[0012] The above described, known filter realizations aredisadvantageous in so far, that they are exclusively available forbalanced line systems or exclusively for unbalanced line systems. Thus,the filter realizations either comprise a balanced input gate and abalanced output gate or an unbalanced input gate and an unbalancedoutput gate. Mixed constructions of coupled resonators and concentrateddevices, like for example inductive devices, are not known.

[0013] Omitting the balanced transformer, the balun, is only known inthe art in combination with surface acoustic wave filters (SAW filters).The relatively extensive and costly SAW filters offer a possibility ofbalancing the unbalanced signals by inserting a physically very short180° degree line in the acoustic area. SAW filters offer a very steepfilter characteristic with average throughput losses of about 2 to 3 dBand have established themselves as receiving filters ahead of thepreamplifiers of the transmit-receive circuits. On the transmission sideof the transmit/receive circuits mainly balanced transformers (Baluns)realized in ceramics are used.

[0014] The drawback of the above-described realizations of band-passfilters known in the art is on the one hand, that band-pass filtershaving discrete inductive devices and capacitors for commercialmicrowave products produced in large amounts are of no interest, as thenumber of required devices is very high and therefore the required areais very large. Further, the losses in the passband are intolerably high.In contrast to that, coupled resonator filters offer better selectionproperties with lower losses than the “interconnected” resonatorfilters, like for example the ladder type structures. The commerciallyvery successful SAW filters are based on an antiquated topology due totheir non-coupled resonator technology.

[0015] For realizing frequency-separating filters, diplexers, the SAWfilters comprise a too high passband attenuation, which lies at about0.7 dB with diplexers. Further, SAW filters are not large-signal stable,so that the signal output by a power amplifier would destroy the filter.After the output of the transmit/receive circuit a SAW filter is tooexpensive.

[0016] A balanced transformer has too little selection propertiescompared to a SAW filter, so that its use is not advantageous either.

[0017] Regarding the realizations for band-pass filters known in the artit can be summarised, that they are too expensive to realize, comprise atoo high loss in the passband and/or a too low selection property.

[0018] U.S. Pat. No. 1,848,221 describes a filter circuit that consistsof a serial inductivity, from a series coil and from two parallelbranches, which respectively consist of a serial circuit, from aninductivity and a capacitor. The filter is connected between an inputcircuit and an output circuit, wherein the coils are coupled to eachother in the parallel branches.

[0019] JP 613706A describes a band-pass filter which comprises an LCseries resonance circuit which is connected in series with another LCseries resonance circuit. An LC parallel resonance circuit is connectedbetween the connection of the two LC series resonance circuits andground. The inductivities used in the LC series resonance circuits arecoupled to each other.

[0020] In the article by Orlov A. T., “Use of active Networks to widenthe spectrum of application of piezoelectric filters”, in “FrequencyControl Symposium, 1994, Proceedings of the 1994 IEEE. InternationalBoston, Jun. 1, 1994, pages 411 to 414” the design of active BAWpiezoelectric filters is described whose properties are not obtainablewith inductivity-free passive piezoelectric filters.

SUMMARY OF THE INVENTION

[0021] It is the object of the present invention to provide an improvedfilter arrangement, which comprises a low passband attenuation, islarge-signal stable and comprises good selection properties.

[0022] The present invention provides a new class of filters, inparticular band-pass filters, that are suitable for symmetric,asymmetric and even mixed-symmetric input and output lines.

[0023] The present invention provides a filter arrangement having afirst resonator circuit connected between a first node'and a secondnode; a second resonator circuit connected between a third node and afourth node, wherein the first resonator circuit and the secondresonator circuit are coupled electro-magnetically; and an inductivedevice connected between the first node and the third node; and afurther inductive device connected between the second node and thefourth node; wherein the first resonator circuit and the secondresonator circuit include line resonators, BAW resonators, SAWresonators, dielectric resonators, quartz resonators and/or opticalresonators.

[0024] In accordance with a second aspect the invention provides afilter arrangement having a first resonator circuit, connected between afirst node and a second node; a second resonator circuit connectedbetween the second node and a third node, wherein the first resonatorcircuit and the second resonator circuit are electro-magneticallycoupled; an inductive device connected between the second node and thefourth node; a third resonator circuit, connected between a fifth nodeand the fourth node; and a fourth resonator circuit connected between asixth node and the fourth node, wherein the third resonator circuit andthe fourth resonator circuit are electro-magnetically coupled; whereinthe first resonator circuit and the second resonator circuit includeline resonators, BAW-resonators, SAW-resonators, dielectric resonators,quartz resonators and/or optical resonators.

[0025] In a preferred embodiment a new approach is pursued, according towhich the concentrated devices are used with uncoupled and coupledlines, whereby very selective band-pass filters arise, that may berealized with a very low number of devices on a very restricted area.

[0026] Apart from these obviously very large advantages for pureband-pass filter functions the present invention further offerstopologies with the use of which filters with an unbalanced input gateand a balanced output gate and/or a balanced input gate and anunbalanced output gate can be realized. These filters do not comprisemore losses than filters with the same input and output gates due totheir concept. With these filters it is possible to save losses andcosts that occur with many applications due to the otherwise necessarybalanced transformers (Baluns).

[0027] In another preferred embodiment the inventory filters arecomprised of at least two coupled lines or resonators and one capacitoror one coil. According to the present invention topologies for coupledserial and parallel resonators are provided.

[0028] In another preferred embodiment of the present invention acircuit topology for a band-pass filter is provided comprisingresonators of which at least two are intercoupled and one or moreinductive devices. According to a further embodiment a circuit topologyfor a band-pass filter is created comprising high-frequency line pairsand one or more inductive devices, wherein the high-frequency line pairscomprise low-impedance and high-impedance lines, wherein at least twohigh-impedance lines are electromagnetically coupled over a partiallength.

[0029] A first embodiment relates to a band-pass filter with an inputand an output of one unbalanced microwave gate each. Behind the inputgate preferably n (n=1, 2, . . . ) Γ elements are provided. Each of theΓ-elements includes one resonator or one high-frequency line pair whichare short-circuited at their ends, wherein the high-frequency line paircomprises a low-impedance and a high-impedance line with an electricallength of smaller than λ/4, wherein λ designates the wavelength of thewave travelling on the high-frequency line. Further, a serial inductivedevice is provided. In front of the output port a further resonator or afurther high-frequency line pair are located, and the further resonatorand/or the further high-frequency line pair are connected to theresonator or the high-frequency line pair at the input port via theserial inductive device. The high-frequency line pair includes alow-impedance and a high-impedance line having an electrical length ofless than λ/4 and terminates with a short-circuit at the end.

[0030] A second embodiment relates to a band-pass filter with an inputand an output of one balanced microwave port each. After the input portn(n=1, 2, . . . ) C-elements are provided. Each of the C-elementsincludes one resonator or one high-frequency line pair each comprisingtwo low-impedance and one high-impedance line having an electricallength of less than λ/4, wherein the low-impedance lines are connectedto the balanced ports and/or lines. For each balanced line branch oneserial inductive device is provided. In front of the output port afurther resonator or a further high-frequency line pair is provided,each comprising two low-impedance and one high-impedance line having anelectrical length of less than λ/4, wherein the low-impedance line isconnected to the balanced lines.

[0031] A third embodiment relates to a band-pass filter with an inputand an output of one unbalanced and one balanced microwave port. Afterthe input port n (n=1, 2, . . . ) C-elements are located. The C-elementsinclude one resonator or one high-frequency line pair, each comprisingtwo low-impedance and one high-impedance line having an electricallength of less than λ/4, wherein the high-frequency line pair isrespectively interconnected to the low-impedance lines with balancedlines. Further, for each balanced line branch a serial inductive deviceis provided. In front of the output port a further resonator or afurther high-frequency line pair is located, respectively comprising twolow-impedance and one high-impedance line having an electrical length ofless than λ/4, wherein with the further high-frequency line pair thelow-impedance lines are respectively connected to the lines.

[0032] A fourth embodiment relates to a band-pass filter having an inputand an output of one unbalanced microwave port each. After the inputport n (n=1, 2, . . . ) L-elements are provided. The L-elements includeone resonator connected in series or one high-frequency line pairconnected in series, comprising one coupled low-impedance and onehigh-impedance line having an electrical length of less than λ/4. Aftera series element an inductive device is located, connected against areference potential, e.g. ground. In front of the output port a furtherresonator or a high-frequency line pair is located, comprising a coupledlow-impedance and a high-impedance line having an electrical length ofless than λ/4 and being inserted serially between the last inductivedevice, e.g. the last coil, and the output.

[0033] A fifth embodiment relates to a band-pass filter having an inputand an output of one balanced microwave port each. After the input portn(n=1, 2, . . . ) mirrored C-elements are located. The C-elements eachinclude two series elements in the form of two resonators orhigh-frequency line pairs, each comprising one coupled low-impedance andone high-impedance line having an electrical length of less than λ/4.Further, an inductive device is provided, connected between the balancedline branches. In front of the output port two further series elementsare located, in the form of two resonators or in the form ofhigh-frequency line pairs each comprising one coupled low-impedance lineand one high-impedance line having an electrical length of less thanλ/4.

[0034] A sixth embodiment relates to a band-pass filter having an inputand an output of one unbalanced and one balanced microwave port. Afterthe input port n (n=1, 2, . . . ) mirrored C-elements are located. TheC-elements each include two series elements in the form of tworesonators or in the form of high-frequency line pairs each comprisingone coupled low-impedance and one high-impedance line having anelectrical length of less than λ/4. Those series elements which are notconnected to the balanced port are connected against a referencepotential, like e.g. ground. An inductive device is connected betweenthe balanced line branches. In front of the output port two furtherseries elements in the form of two resonators or in the form ofhigh-frequency line pairs are located each comprising one coupledlow-impedance line and one high-impedance line having an electricallength of less than λ/4.

[0035] It is an advantage of the present invention, that due to theinventive band-pass filter, which is for example realized as an LDCresonator band-pass filter (LDC=Lumped Distributed Coupled), a highintegrated diplexer (frequency separating filter) is realizable, thatmeets the required properties regarding transmission loss and selectionproperties including a power compatibility of 36 dBm in the handsetdevice.

[0036] A further advantage of the present invention is that it allowssynthesising a band-pass filter that carries out a mode conversionbetween an unbalanced and a balanced power mode apart from itsoutstanding selection properties. If this filter is realized withcoupled lines in a ceramics a very high packing density is achieved onthe one hand due to the low number of line elements and on the otherhand due to the fact, that the lines are coupled and therefore lie veryclose together.

[0037] According to another advantage of the present invention it ispossible to clearly improve resonator filters of high quality, like e.g.SAW filters using the inventive topology, so that the number of requiredresonator is lower with the same blocking isolation and therefore thecosts for realization are-decreased apart from a simultaneous decreaseof transmission losses. If a mode conversion is necessary, no 180° lineis necessary any more according to the present invention. The presentinvention therefore opens the possibility to realize band-pass filterswith mode conversion properties using other high grade resonators, likee.g. using crystal resonators and bulk acoustic wave resonators.

[0038] As no additional lines are necessary according to the presentinvention and as the phase accuracy of a resonator is better than thatof a short acoustic line, in principle lower transmission losses resultfor filters according to the present invention.

[0039] According to another advantage of the present invention atopology for a filter with balanced inputs and outputs is provided.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Preferred embodiments of the present invention are described inmore detail making reference to the enclosed drawings, in which

[0041]FIG. 1 shows an LDC resonator band-pass filter with parallelresonators with an unbalanced input and output port in-line technology;

[0042]FIG. 2 shows an LDC resonator band-pass filter with a balancedinput and output port in-line technology;

[0043]FIG. 3 shows an LDC resonator band-pass filter with parallelresonators having an unbalanced input port and a balanced output portin-line technology;

[0044]FIG. 4 shows a resonator band-pass filter having an unbalancedinput port and a balanced output port with coupled resonators;

[0045]FIG. 5 shows a resonator band-pass filter having series resonatorswith an unbalanced input and output port in-line technology;

[0046]FIG. 6 shows a resonator band-pass filter having series resonatorswith a balanced input and output port in-line technology;

[0047]FIG. 7 shows a resonator band-pass filter having an unbalancedinput port and a balanced output port in-line technology;

[0048]FIG. 8 shows a schematic illustration of the coupledhigh-frequency lines preferably used in the circuits according to thepresent invention;

[0049]FIG. 9A shows a graph of the reflection factor and passageattenuation for a band-pass filter with an impedance of 50 Ω at bothports; and

[0050]FIG. 9B shows the course of reflection factor and passbandattenuation with an impedance of 50 Ω at a first port and of 25 Ω at asecond port.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051]FIG. 1 shows a resonator band-pass filter according to a firstembodiment of the present invention with an unbalanced input and output.The resonator band-pass filter includes a first resonator circuit 102and a second resonator circuit 104. The first resonator circuit 102 isconnected between a first node 106 and a second node 108. The secondresonator circuit 104 is connected between a third node 110 and a fourthnode 112. The resonator band-pass filter further includes an input port114 including a first input port node 116 and a second input port node118. Further, an output port 120 is provided including a first outputport node 122 and a second output port node 124. Further, the resonatorband-pass filter includes an inductive device 126, e.g. in the form of acoil, connected between the first node 106 and the third node 110.

[0052] In the embodiment shown in FIG. 1 the first input port node 116is connected to the first node 106 and the first output port node 122 isconnected to the third node 110. The second node 108, the fourth node112, the second input port node 118 and the second output port node 124are connected to a reference potential 128, e.g. ground.

[0053] The embodiment shown in FIG. 1 illustrates the topologicalconstruction according to the present invention for the case, that theinput 114 and the output 120 are respectively formed by an unbalancedmicrowave port. A large number of line systems used in practice, likee.g. coaxial lines, micro strip lines and strip lines are unbalancedsystems. In FIG. 1 a simple realization is shown using coupledhigh-frequency lines. The first resonator circuit 102 includes a firsthigh-frequency line 130 and a second high-frequency line 132 seriallyconnected-between the first node 106 and the second node 108. The secondresonator circuit includes a third high-frequency line 134 and a fourthhigh-frequency line 136, serially connected between the third node 110and the fourth node 112.

[0054] As is shown in FIG. 1, the line pair comprised of thehigh-frequency lines 130 and 132 is arranged in parallel to the inputport 114 and is therefore connected between the input port node 116 ofthe input port 114 and/or the first node 106 and ground 128, wherein thehigh-frequency lines each comprise an electrical length of less thanλ/4, wherein λ is the wavelength of the waves travelling on the lines.The first high-frequency line 130 comprises an optimized low-impedancewave resistance Z₁₃₀ and is coupled with no other line. The firsthigh-frequency line 130 has a capacitive effect. Instead of therealization of the high-frequency line 130 shown in FIG. 1 it can alsobe implemented as an idling, short tap-line, whose non-idling end isconnected to the node 106, wherein in this case the input of the secondhigh-frequency line 132 is simultaneously connected to the node 106.

[0055] The second high-frequency line 132 has an optimizedhigh-impedance wave resistance Z₁₃₂, which is usually different from thewave resistant Z₁₃₀ of the first high-frequency line. The secondhigh-frequency line 132 is electromagnetically coupled to the fourthhigh-frequency line 136, as it is indicated by the arrow in FIG. 1.

[0056] The third high-frequency line 134 and the fourth high-frequencyline 136 are arranged in parallel to the output port 120 and aretherefore arranged between the output port node 122 of the output port120 and/or the third node 110 and ground 128. Between the two ports 114and 120 the inductive device 126 is arranged between the nodes 106 and110. The inductive device can be realized as a concentrated device, e.g.as a SMD device (SMD=Surface Mounted Device), or as a line device.

[0057] Assuming that the two microwave ports 114 and 120 have the samereference impedance, the third high-frequency line 134 must correspondto the first high-frequency line 130 regarding wave resistance andelectrical length. The same is true for the high-frequency lines 132 and136. In this case the filter circuit must be mirror symmetric withregard to the center axis 138. The inventive filter for impedancetransformation can be used with any reference impedance at both ports114 and 120, wherein in this case the symmetry properties are not givenany more. Using the filter e.g. a high integrated diplexer can berealized.

[0058] In order to achieve better selection properties using the filterof FIG. 1 additional serial inductive devices and line pairs are to beinserted alternatingly, which are interconnected to the referencepotential. However, not all high-impedance lines of the line pairs needto be coupled to each other. The larger the part of coupled lines, thebetter the selection properties.

[0059] Instead of the capacitively operating high-frequency lines 130and 135 concentrated capacitors may also be used, connected between thenodes 106 and/or 110 and the reference potential 128, wherein in thiscase the second high-frequency lines 132 and 136 are directly connectedto the first node 106 and/or 110.

[0060] Instead of the line pairs shown in FIG. 1 also any otherresonators may be used, as far as they can be intercoupled and show aparallel resonance, i.e. a blocking performance, in the passband.Examples of such resonators are surface acoustic wave resonators (SAWresonators), crystal resonators, bulk acoustic wave resonators andsimilar things.

[0061] In FIG. 2 a resonator band-pass filter according to a furtherembodiment of the present invention is shown having a balancedinput/output. In FIG. 2 components described referring to FIG. 1 aredesignated with like reference numerals.

[0062] In the embodiment shown in FIG. 2, the second input port node118, the second node 108, the fourth node 112 and the second output portnode 124 are not connected to a reference potential, in contrast toFIG. 1. Rather, the first input port node 118 is connected to the secondnode 108 and the second output port node 124 is connected to the fourthnode 112. Additionally, a further inductive device 140 is arrangedbetween the second-node 108 and the fourth node 112. Further, the firstresonator circuit 102 additionally includes a fifth high-frequency line142, wherein the high-frequency lines 130, 132 and 142 of the firstresonator circuit 102 are connected between the first node 106 and thesecond node 108. Likewise, the second resonator circuit 104 includes afurther, sixth high-frequency line 144. The high-frequency lines 134,136 and 144 of the second resonator circuit 104 are serially connectedbetween the third node 110 and the fourth node 112.

[0063] In FIG. 2, a band-pass filter is shown, wherein the input port114 and the output port 120 respectively are a balanced microwave port.The filter topology in this case is such, that after the input port 114two line pairs connected in series are provided, comprising thecapacitive high-frequency line 130, the double inductive line 132 andthe additional capacitive line 142, wherein the second high-frequencyline 132 has a length of about λ/2 in this embodiment. Further, theillustrated topology comprises the two serial inductive devices 126 and140 and the additional double line pair consisting of the high-frequencylines 134, 136 and 144, formed by two capacitive lines 134 and 144 andone inductive line 136 with a length of about λ/4, similar to the lines130, 132 and 142. The inductive lines 132 and 136 areelectromagnetically coupled, as it is indicated by the arrow in FIG. 2.The first high-frequency line 130, the fifth high-frequency line 142,the third high-frequency line 134 and the sixth high-frequency line 144are low-impedance lines, whereas the second high-frequency line 142 andthe fourth high-frequency line 136 are high-impedance lines.

[0064] The special thing about the topology illustrated in FIG. 2 is,that it is mirror symmetric with regard to the horizontal center axis146, i.e., that e.g. the inductive devices 126 and 140, for exampleformed by coils, and also the high-frequency lines 130 and 142 and/orthe high-frequency lines 134 and 144 are equal in their electricperformance.

[0065] Assuming that the two ports 114 and 120 comprise an identicalreference impedance, the filter circuit with respect to the verticalcenter axis 138 is mirror symmetric, i.e., the third high-frequency line134 corresponds to the first high-frequency line 130 with regard to itswave resistance and its electrical length. The same is true for thesecond high-frequency line 132 and the fourth high-frequency line 136,for the fifth high-frequency line 142 and the sixth high-frequency line144 and for the coils 126 and 140.

[0066] The filter can be used for an impedance transformation at anyport impedances, wherein in this case the symmetry properties are notgiven any more.

[0067] If it is desired to improve the selection properties of thefilter arrangement in FIG. 2, additional serial inductive devices andline paths are to be provided alternatingly, connected between thebalanced lines 148 and 150, connecting the respective input port nodesto the respective output port nodes.

[0068] It is not necessary for all high-impedance lines to beintercoupled, but the selection properties are the better, the higherthe proportion of intercoupled lines.

[0069] Instead of the capacitively operating high-frequency lines 130and 134 and 142 and 144, concentrated capacitors may also be provided,connected between the signal line and ground. In this case the secondhigh-frequency line 132 and the fourth high-frequency line 136 aredirectly connected to the first node 106 and the second node 108 and/orthe third node 110 and the fourth node 112.

[0070] Instead of the above described intercoupled line pairs, also anyother resonators may be used, as long as they can be intercoupled andhave a parallel resonance (blocking performance) in the passband.

[0071] In FIG. 3 a resonator band-pass filter with a balanced and anunbalanced port is shown, wherein the elements already described in thepreceding Figs. are designated like reference numerals.

[0072] In contrast to FIG. 2, the second input port node 118 of theinput port 114 is connected to a reference potential 128, e.g. ground,in this embodiment of the band-pass filter. The second node 108 is onlyconnected to the second output port node 124 via further inductivedevices 140.

[0073] The band-pass filter shown in FIG. 3 illustrates the case,wherein the input port 114 is an unbalanced port and the output port 120is a balanced port. In this case the filter topology is, that after theinput port 114 two line pairs connected in series are provided as afirst resonator circuit 102 and an additional dual line pair as a secondresonator circuit 104, connected by the two serial inductive devices 126and 140. The line pairs of the first resonator circuit 102 and thesecond resonator circuit 104 each include a first capacitivehigh-frequency line 130 and/or 134, a double inductive high-frequencyline 134 and/or 136 having an approximate length of λ/2, and anadditional capacitive high-frequency line 142 and/or 144.

[0074] The special thing about this topology is, that the filter circuitis mirrorsymmetric with regard to the horizontal center axis 146. Thefilter shown in FIG. 3 can be used for an impedance transformation forany port impedances, wherein the symmetry property with reference to thevertical axis 138 are not given any more in this case.

[0075] If the filter properties, e.g. the selection properties of thisfilter are to be improved, additional serial inductive devices anddouble line pairs are to be provided alternatingly, which are to beconnected between the balanced lines 148 and 150. Although not allhigh-impedance lines need to be intercoupled, the selection propertiesimprove with an increasing number of intercoupled lines.

[0076] Instead of the capacitive and intercoupled lines also any otherresonators may be used that may be intercoupled and comprise a parallelresonance in the passband.

[0077] Such an arrangement is shown exemplary in FIG. 4, where the firstresonator circuit 102 comprises a first resonator 152 and a secondresonator 154, serially connected between the first node 106 and thesecond node 108. The second resonator circuit 104 includes a thirdresonator 156 and the fourth resonator 158, serially connected betweenthe third node 110 and the fourth node 112. Additionally a thirdresonator circuit 160 is provided in the embodiment shown in FIG. 4,including a fifth resonator 162 and a sixth resonator 164, seriallyconnected between a fifth node 166 and sixth node 168. Additionally,further inductive devices 170, 172 are provided, connected between thethird node 110 and the fifth node 166 and/or between the fourth node 112and the sixth node 168. As it is indicated by the arrows in FIG. 4, theindividual resonators are intercoupled. Alternatively, the thirdresonator circuit 160 and the additional inductive devices 170 and 172may be omitted, and in this case an arrangement similar to that of FIG.3 results, wherein the resonator circuit shown in FIG. 4 are used withthe two respective resonators, which are respectively intercoupled,instead of the high-frequency lines shown in FIG. 4.

[0078] The way of intercoupling resonators, which is generally an energycoupling, fully depends on the way of realizing the resonator. If, e.g.,SAW resonators are used, the coupling is effected via acoustic waves.The coupling coefficients are to have an optimized absolute magnitudeand phase value, wherein their value range may be varied by a provisionof coil values.

[0079] Referring to FIG. 1 to 4 embodiments for preferred resonatorband-pass filters were described above, wherein the resonator circuithave a parallel resonance in the passband. In the following, preferredembodiment of resonator band-pass filters are described referring toFIG. 5 to 7, having a serial resonance (passage performance) in thepassband. Referring to FIGS. 5 to 7 so-called complementary structuresare discussed for the filters described referring to FIGS. 1 to 3.

[0080] In FIG. 5 a resonator band-pass filter is shown including a firstresonator circuit 202 and a second resonator circuit 204. The firstresonator circuit is connected between a first node 206 and a secondnode 208. The second resonator circuit 204 is connected between thesecond node and a third node 210. Further, the filter arrangementincludes a fourth node 212. An input port 214 includes a first inputport node 216 connected to the first node 206 and a second input portnode 218. An output port 220 includes a first output port node 222connected to the third node 210, and a second output port node 224.Between the second node 208 and the fourth node 212 an inductive element226 is arranged, e.g. in the form of a coil. The fourth node 212 and thesecond input port node 218 and the second output port node 224 areconnected to a reference potential 228, e.g. ground. The first resonatorcircuit includes a first high-frequency line 230 and a secondhigh-frequency line 232, serially connected between the first node 206and the second node 208. The second resonator circuit 204 includes athird high-frequency line 234 and a fourth high-frequency line 236serially connected between the third node 210 and the second node 208.

[0081] In FIG. 5 the topological construction is shown for the case,that the input 214 and the output 220 are each an unbalanced microwaveport. In FIG. 5 a simple realization is illustrated using coupledhigh-frequency lines. After the input 214 the coupled low-impedancehigh-frequency line 230 is provided, wherein the electrical length ofthe coupled lines are smaller than λ/4. Further, a high-impedance line132 is provided with an electrical length of less than λ/4. In front ofthe output 220 a coupled low-impedance line 234 and a high-impedanceline 236 are provided, wherein their length corresponds to the length ofthe lines 230 and 232. The coil 226 is connected against the referencepotential 228 between the line pairs.

[0082] The high-frequency line 230 has a capacitive effect. The secondhigh-frequency line 232 comprises an optimized wave resistance Z₂₃₂,that is generally different from the wave resistance Z₂₃₀ of the firsthigh-frequency line 230 in that it is of a higher impedance. The secondhigh-frequency line 232 further comprises an optimized length and iselectromagnetically coupled to the fourth high-frequency line 236.

[0083] The high-frequency lines 234 and 236 preferably correspond to thehigh-frequency lines 230 and 232 with respect to their length and theirwave resistance. The high-frequency lines 234 and 236 are locatedbetween the output port 224 and the second node 208 and the inductivedevice 226 is located between the high-frequency lines 232 and 236,interconnected against the reference potential 228 and e.g. realized asa line device or a concentrated device, wherein in this case theinductive device is realized as an SMD device (SMD=Surface MountedDevice).

[0084] Assuming that the two terminal ports 214 and 220, that may e.g.be microwave ports, have the same reference impedance, the firsthigh-frequency line 230 must correspond to the third high-frequency line234 regarding its wave resistance and its electrical-length. The same istrue for the high-frequency lines 232 and 236. In this case the filtercircuit must be mirrorsymmetric with reference to the vertical centeraxis 238.

[0085] The filter circuit can be used for impedance transformation withany reference impedances of the two ports 214 and 220, wherein in thiscase the symmetry properties are not present any more.

[0086] As far as an improvement of selection properties of the filterarrangement shown in FIG. 5 is desired, additional series lines andinductive devices connected to ground need to be inserted alternatingly.Although not all high-impedance lines need to be intercoupled, animprovement of selection properties results with an increasing amount ofintercoupled lines.

[0087] Regarding the high-frequency'lines 230, 232, 234 and 236 providedin the resonator circuit 202 and 204 it is noted, that the lines mayalso be arranged in reverse order compared to FIG. 5. Further, anyresonator circuit may include any number of coupled and/or non-coupledhigh-frequency lines.

[0088] Instead of the lines described in FIG. 5 also any otherresonators may be used, as far as they can be intercoupled and comprisea series resonance (passage performance) in the passband. The topologyshown in FIG. 5 is a so-called complementary structure to the structuredescribed in FIG. 1.

[0089] Referring to FIG. 6 a further embodiment of a filter arrangementis described, wherein both ports 214 and 220 are balanced ports comparedto FIG. 5. The arrangement shown FIG. 6 includes a third resonatorcircuit 280 and a fourth resonator circuit 282 in the balanced line 250in parallel to the balanced line 248. The third resonator circuit 280 isconnected between the fourth node 212 and a fifth node 284, and thefourth resonator circuit 282 is connected between the fourth node 212and a sixth node 286. The fifth node 284 is connected to the secondinput port node 280 and the sixth node 286 is connected to the secondoutput port node 224.

[0090] The third resonator circuit 280 includes a fifth high-frequencyline 288 and a sixth high-frequency line 290 arranged serially betweenthe fifth node 284 and the second node 212. The fourth resonator circuit282 includes a seventh high-frequency line 292 and an eighthhigh-frequency line 294 connected serially between the sixth node 286and the fourth node 212. As is indicated by the arrow in FIG. 6, thehigh-frequency lines 290 and 294 are intercoupled electromagnetically.

[0091]FIG. 6 shows a passband filter arrangement wherein the input andoutput ports 214 and 220 respectively are a balanced port, e.g. amicrowave port. In this case the filter topology is, that after theinput port 214 two double series lines with a length of less than λ/4are connected, herein formed by the lines 230, 232, 288 and 290.Further, the parallel inductive device 226 and two additional line pairsare provided, formed by the high-frequency lines 234, 236, 292 and 294.The special thing about this topology is, that the filter circuit ismirrorsymmetric with regard to the horizontal center axis 246, i.e. thelengths and the wave resistances of the upper lines 230 to 236 are equalto the lengths and wave resistances of the bottom lines 288 to 294.

[0092] Assuming that the two ports 214 and 220 comprise the samereference impedance, the circuit must be mirrorsymmetric referring tothe vertical center axis 238 as well, i.e., the high-frequency line 230corresponds to the high-frequency line 234 with respect to its waveresistance and its electrical length. The same is true for thehigh-frequency lines 232, 288, 290, corresponding to the high-frequencylines 236, 292 and 294 regarding their wave resistances and theirelectrical length.

[0093] The filter arrangement can be used for impedance transformationat any port impedances, wherein in this case the above-mentionedsymmetry properties are not given any more.

[0094] If an improvement of the selection properties of the filterarrangement is to be achieved, additional series lines and inductivedevices connected between the balanced lines 248 and 250 must beinserted alternatingly. Although not all lines need to be intercoupled,the selection properties may be improved with an increasing part ofintercoupled lines.

[0095] The line order in the individual resonator circuit 202, 204, 280and 282 shown in FIG. 6 may also be interchanged. Further, any amountsof coupled and/or incoupled high-frequency lines may be provided in theresonator circuit.

[0096] Instead of the coupled lines described in FIG. 6 also any otherresonators may be used, as far as they may be intercoupled and have aseries resonance (a passage performance) in the passband.

[0097] In FIG. 7, a further embodiment of a resonator bandpass filter ispresented, wherein the input port is an unbalanced port and the outputport is a balanced port.

[0098] Essentially, the structure of the circuit of FIG. 7 correspondsto that of FIG. 6, wherein here, however, in contrast to FIG. 6, thesecond input port node 218 is connected to the reference potential 228,e.g. ground. Further, the fifth node 284 is connected to the referencepotential 228. In other words, only the first balanced line 248 extendsbetween the input port and the output port, in contrast to FIG. 6,wherein the second balanced line 250 extends from the second output portnode 224 to the reference potential 228 via the nodes 286, 212 and 284.

[0099] With the bandpass filter illustrated in FIG. 7, the inputterminal 214 is an unbalanced port and the output terminal 220 is abalanced port. The filter topology in this case is, that after the inputport 214 two line pairs (high-frequency lines 230, 232, 288, 290), aparallel inductive device 226 and two further line pairs (high-frequencylines 232, 236, 292, 294) are connected.

[0100] The special thing about this topology is, that the filter circuitis mirrorsymmetric with regard to the horizontal central axis 246, i.e.,the upper lines 230 to 236 and the lower lines 288 to 294 are equalregarding their electrical performance, their wave length and theirelectrical lengths.

[0101] Assuming that the impedance of the input port 214 corresponds tohalf of the impedance of the output port 220, the high-frequency line230 must correspond to the high-frequency line 234 regarding its waveresistance and its electrical length. The same is true forhigh-frequency lines 232, 288 and 290, that need to correspond to thehigh-frequency lines 236, 292 and 294 with regard to the electricalproperties. The circuit must also be symmetric regarding the verticalcenter axis 238.

[0102] Assuming any port impedances at the input port 214 and 220 thefilter can be used for impedance transformation, wherein in this casethe above mentioned symmetry properties are not given any more.

[0103] In order to improve the filter properties, like e.g. theselection properties, additional series lines and inductive devices,connected between the balanced lines 248 and 250 must be insertedalternatingly. Although not all high impedance lines need to beintercoupled, the selection properties of the filter may be improveddepending on the number of intercoupled lines.

[0104] As with the embodiments described according FIGS. 5 and 6, theorder of the high-frequency lines can be reversed at the resonatorcircuit. Further, the resonator circuit may comprise any number ofcoupled and/or uncoupled high-frequency lines.

[0105] Instead of the coupled lines 230 to 236 and 288 to 294 also allother resonators may be used, as far as they can be coupled and comprisea series resonance (passage performance) in the passband.

[0106] With regard to the embodiment described above in more detailreferring to FIG. 1 to 7 it is noted, that it is the case with allfilter arrangements that both possible directions may be used for anelectromagnetic coupling. Often, these couplings are marked with a pointat the device (line or coil). In this case the two points may lie leftand/or right or inside and/or outside.

[0107] In general it is valid for all above described embodiments, thatthe input port 114, 214 and the output port 120, 220 may always beinterchanged. Instead of the capacitively operating lines concentratedcapacities in the form of chip capacitors or similar things may be used,wherein in the case of the embodiment described referring to FIGS. 1 to3 these capacitors are connected between the signal line and ground, andin the case of the embodiments described referring to FIGS. 5 to 7 thesecapacitors are interconnected serially.

[0108] It is noted, that the described input/output ports must not bemicrowave ports, but that any port is included in the generally definednetwork theory. If piezoelectric resonators are used, the inventivetopology may also be used with low frequencies. If optical resonatorsare used the concept may also be used in optics.

[0109] The construction of serial lines and/or serial resonators andparallel lines and/or parallel resonators may also be combined togetherin a chain circuit from the minimal construction stage of the abovedescribed basic cells with three elements. Some filters comprise aserial and a parallel resonance that lie very close together, so that aresonator cell for both cases may be used for such filters.

[0110] The coupled high-frequency lines 232 and 236 used in theembodiments described in FIGS. 1 to 3 may be halved and the inductivedevice 246 described in FIGS. 5 to 7 may be replaced by two inductivedevices with twice this value, and then they are contacted with groundfor the balanced ports in the symmetry plane 246 for the circuits,whereby the balanced line systems are put in a symmetry against ground.

[0111] The capacitive lines of the resonator, that is the lines 130,230, 134, 234, 142, 288 and 144, 292 are advantageously realized by twooverlapping, low impedance lines.

[0112]FIG. 8 schematically shows a coupled high-frequency line. Betweena first port formed by the terminals n1 and n2 and a second port formedby the terminals n3 and n4 the coupled high-frequency line 300 isarranged. The high-frequency line 300 includes a first line 302 and asecond line 304 arranged in parallel, both arranged in an isolatedhousing 306 connected to a reference potential, e.g. ground. The firstline 302 is connected between the terminal n1 and the terminal n4, andthe second line 302 is connected between the terminal n2 and theterminal n3. The lines 302 have a length designated “P”, a widthreferred to as “W” and a vertical distance from the walls formed by thehousing 306, referred to as S. The lengths and widths of the lines 302and 304 are preferably equal.

[0113] The capacitive lines 130, 230, 134, 234, 142, 288 and 144, 292 ofthe resonators are realized through lines 302 and 304. With the coupledlines the resonators are high-frequency lines 132, 232 and 290 and areformed by the first line 302 and the high-frequency lines 136, 236 and294 by the second line 304.

[0114] According to an embodiment the band-pass filters describedreferring to FIG. 1 to 7 have a passband of approximately 1.7 GHz toapproximately 2.0 GHz. The resonators comprise the lines describedreferring to FIG. 8 and their elements are optimized for this frequencyarea.

[0115] According to an embodiment of the present invention, wherein sameimpedances are present at the port of the circuits, e.g. 50 Ω, theinductivity of the inductive devices used lie between 1 nH and 10 nH,the length of the capacitive lines lies between 0.1 mm and 0.8 mm, thewidth of the coupled lines lies between 0.1 mm and 0.8 mm, the length ofthe coupled lines lies between 3.0 mm and 8 mm, and the width of thecoupled lines lies between 0.06 mm and 0.2 mm. The distance of the linesfrom the wall preferably is about 0.02 mm. The capacitive lines at theinput and at the output have the same dimensions in this case.

[0116] According to a further embodiment of the present inventionwherein different impedances are present at the ports of the circuits,e.g. 50 Ω and 25 Ω, the inductivity of the inductive devices used liesbetween 1 nH and 10 nH, the length of the capacitive lines lies between0.1 mm and 0.8 mm, the width of the coupled lines lies between 0.1 mmand 0.8 mm, the length of the coupled lines lies 3.0 mm and 8 mm and thewidth of the coupled lines lies between 0.06 mm and 0.2 mm. The distanceof the lines from the wall preferably is about 0.02 mm. The capacitivelines at the input and at the output do not comprise the same dimensionsin this case.

[0117]FIG. 5 is again referred to as an example. It is assumed that thecapacitive lines 230 and 234 are respectively formed by one of the lines300 shown in FIG. 8. The coupled lines 232, 236 are formed by a commonline 300 (FIG. 8).

[0118] Assuming same impedances of 50 Ω at the input port 214 and a atthe output port 220 the elements comprise the following optimized valuesfor a passband from 1.71 GHz to 1.99 GHz:

[0119] The widths “W” of the lines of the first high-frequency line pair230 and the widths “W” of the second high-frequency line pair 234 arethe same and amount 0.41296 mm. The length “P” of the lines 302, 304 ofthe first high-frequency line pair 230 and the length “P” of the secondhigh-frequency line pair 234 are the same and amount 0.41296 mm.

[0120] The width “W” of the high-frequency line 232 and the width “W” ofthe second high-frequency line pair are the same and amount to 0.06552mm. The length “P” of the high-frequency line 232 and the length of thesecond high-frequency line 236 are the same and amount to 4.169 mm.

[0121] The distance of all lines to the wall is 0.02 mm.

[0122] The inductive device 226 has an inductivity of 3.1891 nH.

[0123] The above values of the elements result from optimising thereflection factor and the passband attenuation in the passband of thefilter, with S₁₁=−21 dB and S₂₁=0 dB in the passband, and S₂₁=−22 dBoutside the passband. FIG. 9A shows the course of the reflection factor(S₁₁) and the passband attenuation (S₂₁) over the frequency area from 0GHz to 8 GHz.

[0124] Assuming an impedance of 50 Ω at the input port 214 and animpedance of 25 Ω at the output port 220 (or the other way round), theelements have the following optimized values for a passband from 1.71GHz to 1.99 GHz (impedance transformation):

[0125] The widths “W” of the lines of the first high-frequency line pair230 amount to 0.49379 mm, the widths “W” of the lines of the secondhigh-frequency line pair 234 amount to 0.53553 mm. The lengths “P” ofthe lines 302, 304 of the first high-frequency line pair 230 amount to0.49379 mm and the lengths “P” of the second high-frequency line pair234 amount to 0.53553 mm.

[0126] The width “W” of the high-frequency line 232 and the width “W” ofthe second high-frequency line pair are equal and amount to 0.10583 mm.The length “P” of the high-frequency line 232 and the length of thesecond high-frequency line 236 are equal and amount to 3.576 mm.

[0127] The distance of all lines to the wall amount to 0.02 mm.

[0128] The inductive device 226 has an inductivity of 2.5432 nH.

[0129] The above values of the elements result from an optimisation ofthe reflection factor and the passband attenuation in the passband areaof the filter, with S₁₁=−21 dB and S₂₁=0 dB in the passband, and S₂₁=−22dB outside the passband area. FIG. 9B shows the course of the reflectionfactor (S₁₁) and the passband attenuation (S₂₁) over the frequency areafrom 0 GHz to 8 GHz.

[0130] While this invention has been described in terms of severalpreferred embodiments, there are alterations, permutations, andequivalents which fall within the scope of this invention. It shouldalso be noted that there are many alternative ways of implementing themethods and compositions of the present invention. It is thereforeintended that the following appended claims be interpreted as includingall such alterations, permutations, and equivalents as fall within thetrue spirit and scope of the present invention.

What is claimed is:
 1. A filter arrangement, comprising a firstresonator circuit connected between a first node and a second node; asecond resonator circuit connected between a third node and a fourthnode, wherein the first resonator circuit and the second resonatorcircuit are coupled electro-magnetically; and an inductive deviceconnected between the first node and the third node, wherein a furtherinductive device is connected between the second node and the fourthnode; wherein each of the first resonator circuit and the secondresonator circuit includes at least one of a line resonator, a BAWresonator, a SAW resonator, a dielectric resonator, a quartz resonatorand an optical resonator.
 2. The filter arrangement according to claim1, wherein the first resonator circuit and the second resonator circuitdefine a passband, have a parallel resonance in the passband, and arecoupled.
 3. The filter arrangement according to claim 1, furthercomprising a first port with a first port node connected to the firstnode, and a second port node connected to the second node, a second portwith a third port node connected to the third node, and a fourth portnode connected to the fourth node.
 4. The filter arrangement accordingto claim 1, further comprising a first port with a first port nodeconnected to the first node, and a second port node connected to areference potential, a second port with a third port node connected tothe third node, and a fourth port node connected to the fourth node. 5.The filter arrangement according to claim 1, wherein the first resonatorcircuit includes a first capacitive element and a first line element,and wherein the second resonator circuit includes a second capacitiveelement and a second line element.
 6. The filter arrangement accordingto claim 5, wherein the first capacitive element and the secondcapacitive element are formed by at least on of a capacitorinterconnected against a reference potential, a capacitive line, and anopen tap line.
 7. The filter arrangement according to claim 5, whereinthe first resonator circuit comprises a further capacitive elementconnected between the first line element and the second node, andwherein the second resonator circuit comprises a further capacitiveelement connected between the second line element and the fourth node.8. The filter arrangement according to claim 7, wherein the additionalcapacitive element is a capacitive line.
 9. The filter arrangementaccording to claim 1, wherein the first resonator circuit and the secondresonator circuit each comprise at least one resonator.
 10. The filterarrangement according to claim 9, wherein the inductive device includesa first inductive device connected between the first node and a fifthnode, and a second inductive device connected between the third node andthe fifth node; wherein the additional inductive device includes a thirdinductive device connected between the second node and a sixth node, anda fourth inductive device connected between the fourth node and thesixth node; and wherein a further resonator circuit coupled to the firstresonator circuit and to the second resonator circuit is connectedbetween the fifth node and the sixth node.
 11. The filter arrangementaccording to claim 10, wherein the first, the second and the furtherresonator circuit includes a plurality of coupled resonators.
 12. Afilter arrangement, comprising a first resonator circuit, connectedbetween a first node and a second node; a second resonator circuitconnected between the second node and a third node, wherein the firstresonator circuit and the second resonator circuit areelectro-magnetically coupled; and an inductive device connected betweenthe second node and the fourth node; a third resonator circuit connectedbetween a fifth node and the fourth node; and a fourth resonator circuitconnected between a sixth node and the fourth node, wherein the thirdresonator circuit and the fourth resonator circuit areelectro-magnetically coupled, and wherein each of the first resonatorcircuit and the second resonator circuit include at least one of a lineresonator, a BAW-resonator, a SAW-resonator, a dielectric resonator, aquartz resonator and an optical resonator.
 13. The filter arrangementaccording to claim 12, wherein the first resonator circuit and thesecond resonator circuit define a passband, are coupled, and have aserial resonance in the passband.
 14. The filter arrangement accordingto claim 12, further comprising a first port with a first port nodeconnected to the first node, and a second port node connected to thefifth node, and a second port with a third port node connected to thethird node, and a fourth port node connected to the sixth node.
 15. Thefilter arrangement according to claim 12, further comprising a firstport with a first port node connected to the first node, and a secondport node connected to a reference potential, a second port with a thirdport node connected to the third node, and a fourth port node connectedto the fifth node.
 16. The filter arrangement according to claim 12,wherein each resonator circuit includes one capacitive element and oneline element.
 17. The filter arrangement according to claim 16, whereinthe capacitive element is formed by a serially connected capacitor, by acapacitive line or by an open tap line.
 18. The filter arrangementaccording to claim 12, wherein the resonator circuit each comprise atleast one resonator.